As pressure-sensitive adhesive use is made inter alia of solvent-free polyacrylates or aqueous polyacrylate dispersions.
In the case of aqueous polymer dispersions a drying is necessary for the purpose of removing the water.
The desire is therefore for solvent-free pressure-sensitive adhesives.
Radiation-crosslinkable polymers and their use as an adhesive, e.g., as a hotmelt pressure-sensitive adhesive, are known, for example from EP-A-246 848 or EP-A-377 199.
In the case of radiation-crosslinkable pressure-sensitive adhesives (PSAs) the cohesion, i.e., the internal strength of the adhesive layer, after the adhesive has been coated onto a backing, is achieved by means of subsequent, photochemically induced crosslinking.
Adhesion (sticking to the substrate) and cohesion are divergent performance properties. Measures which bring about an improvement in the adhesion generally lead at the same time to a deterioration in the cohesion, and vice versa.
The desire is therefore for measures which improve both the cohesion and the adhesion, but at least do not improve one of the two properties at the expense of the other. Also of importance is a high level of thermal stability on the part of the adhesive bond; that is, the bond should withstand mechanical loads even at relatively high temperatures. Particularly in the case of industrial applications, and especially with adhesive assembly tapes, high thermal stability is often critical to the use of an adhesive.
Particular applications require high layer thicknesses of the pressure-sensitive adhesive.
Radiation-crosslinkable pressure-sensitive adhesives often have the drawback here that effective adhesion and cohesion are no longer achieved at high layer thicknesses.
WO 2004/081133, EP-A 1 578 823, EP-A 628 616 and EP-A 1 469 036 disclose mixtures of radiation-curable polyacrylates and oligomers containing two or more acrylic groups, and also disclose the use of these mixtures as pressure-sensitive adhesives.
It was an object of the present invention to improve the cohesion in conjunction with effective or sufficient adhesion, and also an effective heat stability, not least at high layer thicknesses of the pressure-sensitive adhesive.